Temperature gauge fluctuating

[72turbojet]

Recently the temp gauge in my 72 has been fluctuating from normal temp to a hotter reading. My first instinct was the thermostat so I replaced it yet still had the same problem. I checked the temp with a temp gun when the gauge showed both hot and normal reading and the engine is staying at a consistent upper 180s. I also tried a new sensor and have had no success so far. Any ideas what could be causing this? I know it's not my headers because it was fine for a long time until just recently and nothing looks out of the ordinary under the hood